1. Field of the Invention
The present invention relates to a method of operating a cellular communications system in which secondary stations are able to roam in and out of the radio coverage area (or "cell") of one or more fixedly sited primary stations, such stations regularly transmitting idle beacon signals which are detectable by in range secondary stations.
The invention also relates to such a communications system and to a secondary station for use therein.
2. Description of the Related Art
For convenience of description the present invention will be described with reference to DECT (Digital European Cordless Telecommunications) system which comprises a plurality of time division duplex frequency channels which are accessed using a time division multiple access (TDMA) protocol. More particularly DECT comprises, for voice communication, one or more geographically separated primary or fixed base stations each having radio transceiving means defining a cell and a land line connection to the public service telephone network (PSTN), and one or more secondary or transportable, for example hand portable, stations having radio transceiving means which are able to communicate by way of a radio link with an in-range base station. The European Telecommunications Standards Institute (ETSI) currently proposes that DECT shall have ten radio carrier channels but this number may be increased later if there is sufficient demand. Each radio channel is at a particular carrier frequency, and is divided into time frames of 10 milliseconds duration. Each frame is divided into 24 equal time slots (or physical channels) which are paired to establish 12 time division duplex (TDD) channels termed duplex voice channels. The TDD arrangement is such that the nth and the (n+12)th time slots, where n is an integer between 1 and 12, are the forward and reverse physical channels constituting a complete duplex voice channel. Each such pair of physical channels is capable of carrying one duplex digitised speech conversation, or data at a rate of 32 kbits/sec. As the framing structure of all of the radio carrier channels is synchronised, this means that the correspondingly numbered physical channels in each of the radio carrier channels coincide in time.
In setting up a voice call between a primary and a secondary station, a duplex voice channel is assigned to the transaction. The assignment of the duplex voice channel in any of the radio carrier channels is by the method of dynamic channel allocation whereby a secondary station, taking into account its radio environment as determined by monitoring the average interference in each of the 120 pairs of physical channels, negotiates with the primary station for access to the best duplex voice channel currently available under the control of the primary station.
Currently the DECT specification requires base stations to continue transmissions on their last active forward physical channel to provide the system functions of broadcasting the basic system information, base station identity, and a frequency and frame timing reference. For convenience of description such transmissions will be called "normal idle beacon" transmissions.
A secondary station, once locked to a primary station, remains in synchronism with the framing structure of the primary station but can be powered down or "sleep" for relatively long periods. However the secondary station is awakened for the occurrence of the primary station's active forward physical channel in one frame (10 mS) in every 16 frames (or 160 mS) in order to be capable of receiving any paging transmissions addressed to it. This constitutes one out of every 16.times.24=384 physical channels. By having such an arrangement for the reception of possible paging signals, the secondary station therefore has a duty cycle of the order of 1 in 384, which is effective in reducing the power consumption. However since the secondary station need be active for only a portion of the physical channel in order to receive any paging message, a duty cycle of 1 to 2000 can be achieved with this arrangement.
In the case of a secondary station which is out-of-lock, it wakes up approximately every 200 seconds and tries to detect a system onto which it can lock by scanning all the carrier channels in succession, each carrier channel being monitored for the duration of one frame which in the case of 10 carrier channels will take 100 mS. Such a search of the carrier channels not only upsets the duty cycle of the secondary station, which if there are no paging messages is of the order of 1 in 2000 for a locked secondary station, but also reduces its battery life.
An object of the present invention is to make scanning of the carrier channels more efficient.
According to one aspect of the present invention there is provided a method of operating a cellular radio communications system which comprises a primary station and a plurality of secondary stations, communication between the primary and secondary stations being in accordance with a time division multiplex protocol in which each of a plurality of carrier channels is divided into time frames and each frame is divided into a plurality of physical channels, and wherein an out-of-lock secondary station is powered up to periodically scan the physical channels in each carrier channel in order to detect those physical channels in which transmissions are taking place. The method is characterised in that the scanning sequence of the secondary station is controlled so that during any one scan of the sequence consecutive physical channels which are located in one of a series of non-adjacent carrier channels are scanned.
According to a second aspect of the invention there is provided a cellular radio communications system comprising a primary station and a plurality of secondary stations, communication between the primary and secondary stations being in accordance with a time division multiplex protocol in which each of a plurality of carrier channels is divided into time frames and each frame is divided into a plurality of physical channels and, wherein an out-of lock secondary station is powered up to periodically scan the physical channels in each carrier channel in order to detect those physical channels in which transmissions are taking place. The system is characterised in that the secondary station comprises control means for controlling the scanning sequence so that during any one scan of the sequence consecutive physical channels which are located in one of a series non-adjacent carrier channels are scanned.
According to a third aspect of the invention there is provided a secondary station for use in the aforesaid system, comprising transmitting and receiving means and control means for controlling the transmitting and receiving means. The control means is responsive to the secondary station being out-of-lock to initiate scanning of all the available physical channels in the respective carrier channels. The scanning sequence is controlled so that during any one scan of the sequence consecutive physical channels which are located in one of a series of non-adjacent carrier channels are scanned.
The invention is based on the fact that the characteristics of a transmitted signal enable its presence in an adjacent carrier channel to be determined when carrying out an earlier scanning sequence. As a result, a channel scanning protocol can be devised which enables some but not all the carrier channels to be scanned at any one time to thereby obtain a good indication of the presence of a transmitting primary station not only in the carrier channels being scanned but also in the carrier channels adjacent to those being scanned.
The respective scanning sequences may be all the odd numbered carrier channels and then all the even numbered carrier channels.
These sequences may be further subdivided into four scan sequences comprising, respectively, the forward transmissions in the odd numbered carrier channels, the forward transmissions in the even numbered carrier channels, the reverse transmissions in the odd numbered channels and the reverse transmissions in the even numbered channels. Other variations are also possible.
In one embodiment each scanned physical channel is examined for substantially its whole duration. However an alternative method is to monitor the signal strength for only a fraction, for example one eighth, of the period of each physical channel. Such a fraction may be say 50 .mu.s out of 416 .mu.s. However care must be taken to ensure that said fraction does not occur during a guard space.
In a development of the system, each primary station transmits a system identity code which may be used by a secondary station to obtain information about its neighbouring systems, which information is stored. When an out of range secondary station detects a neighbouring system, it can therefore anticipate that it is approaching its own local system and as a consequence it may increase its frequency of scan so that it is able to initiate a call within the guaranteed maximum connection time of 1.5 seconds specified by most telephone authorities.